An ultrasonic wave propagates through a solid and various other media, and therefore, has been used in a wide variety of fields including measurement, evaluation of physical properties, engineering, medicine and biology.
The propagability of an ultrasonic wave through a medium is represented as acoustic impedance. Generally speaking, at an interface between two types of media with significantly different acoustic impedances (such as a gas and a solid), most of the ultrasonic wave that has been propagated through one of those two media will be reflected, and the ultrasonic wave cannot be transmitted to the other medium with high efficiency.
An ultrasonic vibrator is used extensively to detect an ultrasonic wave and is often made of a piezoelectric body such as a ceramic. That is why if an ultrasonic wave that has been propagated through a gas needs to be detected by an ultrasonic vibrator, most of the ultrasonic wave propagated is reflected from the surface of the ultrasonic vibrator and only a portion of that ultrasonic wave is detected by the ultrasonic vibrator. For that reason, it is usually difficult to detect an ultrasonic wave with high sensitivity. In transmitting an ultrasonic wave from an ultrasonic vibrator into the air, the efficiency will also decrease due to the reflection. That is why particularly when an ultrasonic wave is used to measure a distance or a flow rate or to sense an object, it is one of the most important problems to detect the ultrasonic wave with high sensitivity.
In order to overcome this problem, Patent Document No. 1, for example, discloses an ultrasonic transducer that can detect an ultrasonic wave, propagating through an environmental fluid such as a gas, with high sensitivity by utilizing the refraction of the ultrasonic wave and that can transmit ultrasonic waves through an environmental fluid in a broad frequency range. Hereinafter, such an ultrasonic transducer will be described.
As shown in FIG. 14, the conventional ultrasonic transducer 201 includes an ultrasonic vibrator 202 and a propagation medium 203, which is arranged on a first surface area 231 that is the transmitting, and receiving surface of the ultrasonic vibrator 202. The environment surrounding the ultrasonic transducer 201 is filled with an environmental fluid 4, through which an ultrasonic wave propagates in the direction indicated by the arrow 205 so as to reach a second surface area 232 of the propagation medium 203. An ultrasonic transducer of this type is called a “refraction propagation type ultrasonic transducer”.
As the propagation medium 203, a substance that propagates an ultrasonic wave at a lower acoustic velocity than the ultrasonic wave propagating through the environmental fluid 4 and that has a higher density than the environmental fluid 4 is selected. Patent Document No. 1 discloses a dry gel material with a silica skeleton as such a substance. The silica dry gel is a material that can have its acoustic velocity and density adjusted by modifying the conditions for the manufacturing process. For example, in the where the environmental fluid 4 is an air, the material of the propagation medium 203 may be selected such that the medium 203 has a density of 200 kg/m3 and an acoustic velocity of 150 m/s.
Suppose the angle formed between the first and second surface areas 231 and 232 is identified by θ1 and the angle defined by the ultrasonic wave propagating direction 205 with respect to a normal to the second surface area 232 is identified by θ2. In that case, by choosing appropriate angles θ1 and θ2, the reflection of the ultrasonic waves from the second surface area 232 can be reduced to substantially zero. As a result, an ultrasonic transducer with high transmission and reception sensitivity is realized.
According to Patent Document No. 1, in this case, the angles θ1 and θ2 should be approximately 26 degrees and approximately 89 degrees, respectively, and the ultrasonic wave transmitted from the ultrasonic vibrator 202 goes substantially parallel to the second surface area 232. Or an ultrasonic wave that has come substantially parallel to the second surface area 232 is incident on the propagation medium 203 without being reflected from it and then detected by the ultrasonic vibrator 202. As a result, an ultrasonic wave can be introduced from a medium with extremely small acoustic impedance such as the air into a propagation medium with high efficiency or can be radiated from the propagation medium into the air with high efficiency. In this manner, ultrasonic waves can be transmitted and received with high sensitivity.                Patent Document No. 1: Pamphlet of PCT International Application Publication No. 2004/098234        